A 2016 JAMA Internal Medicine report based on the Nurse’s Health Study, which includes 237,000 women, found that replacing saturated and/or trans fats with certain unsaturated ones reduced mortality up to 27 percent. The nurses’ study has also yielded papers linking saturated fat with cardiovascular risk. Even experts like Dean Ornish, MD, who believes fats should be confined to 10 percent of calories, now admit all fats aren’t created equal; unlike Teicholz’s, his camp still spurns saturated fat.

The USDA guidelines are meant to settle these arguments, and the latest batch, formulated in 2015, include a cap on added sugar intake (10 percent of calories) and remove limits on cholesterol. (“Eggs are no longer the villain they were,” says Rafael Pérez-Escamilla, PhD, of the Yale School of Public Health, who served on the Dietary Guidelines Advisory Committee. It’s perhaps similar to what’s happened with fat: “People conflated dietary cholesterol with blood cholesterol,” Krauss said.) The USDA also abolished the ceiling on total fat but suggested limiting saturated fat to 10 percent of calories, and still recommended low- or fat-free dairy and lean meats. “They’re not calling it a low-fat diet anymore,” Teicholz said, “but they’re still counseling it.”

There tends to be so little breathing room between the factions that they don’t even agree on what’s changed in our diet since 1977, when the USDA guidelines were introduced, and when—incidentally or coincidentally, depending on whom you ask— obesity rates began climbing in the United States. The experts I spoke to were so attached to their own positions that it seemed like they existed in opposing realities. Ornish, in a 2015 New York Times op-ed, stated that for decades, Americans had been eating more fat and meat—per the USDA, “67 percent more added fat, 39 percent more sugar, and 41 percent more meat in 2000 than they had in 1950.” Opponents countered, as in a Scientific American article titled almost parodically “Why Almost Everything Dean Ornish Says About Nutrition Is Wrong,” that what matters is we’ve been eating more of everything—23 percent more calories a day than in 1970, according to the Pew Research Center, with the percent of calories from protein and fat dropping, and those from carbs increasing.

It’s tempting, when presented with all of this, to assume the medical establishment doesn’t know enough to arrive at any definitive conclusions— and some scientists agree. A systemic review of research on heart disease and fat published in 2015 in the journal Open Heart had this to say about the USDA’s initial ’77 recommendations to reduce fat: “It seems incomprehensible that dietary advice was introduced for 220 million Americans…given the contrary results.” In an opinion piece in the Annals of Internal Medicine, Steven Nissen, MD, a cardiologist at the Cleveland Clinic, called the latest guidelines “a nearly evidence-free zone.”

The 2015 guidelines had inspired an unprecedented amount of concern. During a public comment period prior to their release, they received 29,000 comments, as opposed to 2,000 in 2010. Congress subsequently earmarked $1 million for the National Academy of Medicine to investigate whether the end result of the process is sufficiently “science based.”

It’s gotten better, said Walter Willett, MD, a Harvard Medical School professor who’s the most cited nutritionist in the world, but when he started off in the 1970s, the disagreements people had were “almost like the religious wars of the Middle Ages.” He’s far from the only one to use theological terminology to describe the field. The USDA guidelines are the “bible,” Teicholz told me. Nissen, in his Annals op-ed, called advocates of different diets “cult-like.” Sugar is the “devil incarnate,” said Robert Lustig, MD, a neuroendocrinologist at UCSF. “Maybe it’s that food has become a way, in a postreligious time, for people to find meaning and an outlet for their desire to change the world,” Teicholz mused. “And I’m sympathetic to that!” The problem, for her, is that this means many cling to assumptions beyond what scientific skepticism demands. She’d hoped her book would motivate more thorough research. Instead, “people just tried to shut my work down,” she said. “It’s like science itself has entered a postfactual state.”

This is the rare statement about which both sides concur. “Perhaps you have recently heard you can’t always trust the news to be real rather than fake,” wrote David Katz, MD, a vocal proponent of a diet featuring plants, some lean meats, and few saturated fats, on the website Verywell .com in December. “If that is true about the fate of the presidency in America, it is no less true about the fare on the typical American plate.”

“It’s like science itself has entered a postfactual state.”

One of Ornish’s claims to fame is that he counseled Bill Clinton after he developed heart dis- ease, resulting in the former president’s shrinking physique. But it turns out Bill and Hillary have also sought medical advice from Mark Hyman, MD, director of the Cleveland Clinic Center for Functional Medicine and author of the best-seller Eat Fat, Get Thin. Earnest and fit, Hyman has the glow of a man who’s spent more than a few afternoons on the Dr. Oz stage (he’s appeared well over a dozen times on Oz’s show). Last November, before giving a talk at ABC Carpet & Home, an upscale, boho NYC furniture store, he sat amid votive candles and tastefully weathered sculptures of spiritual deities, explaining how we’d arrived at this confused point.

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Since randomized clinical trials are often prohibitively expensive, the nutrition world relies on observational studies—usually surveys, he said, which is where the trouble begins. Do you remember what you had for dinner last Tuesday? How much cheese you ate in the last month? It’s led to a research pool so vast that it can be used to support virtually any theory, and the correlations found in such studies are often mistaken for causation. “I always joke, you can do a study of sexually active women over 55 and conclude sex doesn’t lead to pregnancy,” Hyman said. There is also the “healthy-user bias,” whereby subjects who choose to engage in one supposedly healthy activity are likely to be engaging in others. Researchers control for some behaviors, but in regard to obesity, since rates began growing, “everything has changed,” obesity researcher William Dietz, MD, told the New Yorker last September. “Everything on the dietary side. Everything on the physical-activity side. Everything.” How do you control for everything?

Then there are the economic interests, Hyman said, “which interfere with people’s ethics and judgments about what’s true.” After the low-fat diet swallowed up the guidelines, the food industry responded with thousands of new products, but since removing the fat from food often leaves it tasting bland, manufacturers pumped it with sugar. Now the conventional wisdom is turning against sugar, and the new nutritional labels require manufacturers to list added sugars, but the unit of measurement will be grams, rather than the more transparent teaspoons. Why? “You know, the sugar industry loves Congress,” Pérez-Escamilla says. Put less obliquely, sugar companies spent $13.5 million lobbying federal legislators during the last two election cycles, and the business prefers grams (presumably because the less people can picture how much sugar is in a cookie, the more likely they’ll be to purchase it). The influence that agribusiness wields on government policy also helps explain why, whenever new (solid) science does make it into the USDA guidelines, “subsidy policies contradict almost everything in them,” says Pérez-Escamilla. (Between 1995 and 2012, the U.S. government provided $292.5 billion in farm subsidies, but only 1 percent of that went to fruits and vegetables, while 50 percent was funneled toward corn, wheat, and soy.)